Radio frequency (RF) receivers are used in a wide variety of applications such as cellular or mobile telephones, cordless telephones, personal digital assistants (PDAs), computers, radios and other devices that transmit or receive RF signals. In processing an RF signal, digital processing circuitry in an RF receiver may induce spurious frequency content onto the RF signal.
FIG. 1 is a diagram illustrating one embodiment of spurious frequency content in a digital signal 24 such as an RF signal. Circuitry 10 (e.g., an inverter as shown) receives a digital input signal 16 at an input terminal 12 and outputs an output signal 24 at an output terminal 14. As shown in a graph 18, input signal 16 has desired frequency content at an input frequency f1 and at harmonics of the input frequency (e.g., 2f1, 3f1, 4f1, etc.). A voltage supply 20 (VDD) that provides power to circuitry 10, however, has undesired frequency content at a frequency f2 and at harmonics of the frequency (e.g., 2f2, 3f2, 4f2, etc.). The undesired frequency content on voltage supply 20 causes spurious frequency content on output signal 24 at frequencies f1+f2, f1−f2, f1+2f2, f1−2f2, . . . , 2f1+f2; 2f1−f2, 2f1+2f2, 2f1−2f2, . . . , etc. The spurious frequency content is shown by the dotted arrows in a graph 26 around the solid arrows that indicate the frequency content at the input frequency f1 and the harmonics of the input frequency. The spurious frequency content in digital signal 24 occurs as a result of AM-to-PM conversion of voltage supply 20 in circuitry 10 and causes phase jitter in output signal 24 so that signal transitions in output signal 24, such as signal transitions 24A and 24B, may be undesirably shifted in time.